Power transmitting parts such as gears that are used for automotive transmissions and pulleys for belt-type continuously variable transmissions (CVTs) have conventionally been produced as described below. A steel material of low-alloyed steel for machine structural use specified in JIS G4053 (2008) is formed into a predetermined shape by forging, cutting or the like, is subjected to surface hardening treatment such as carburizing-quenching or carbonitriding-quenching, and thereafter is further tempered.
In recent years, a demand for improved fuel economy of motor vehicles has become more stringent. Accordingly, in order to realize a reduction in weight of vehicle body which directly leads to the improvement in fuel economy, further reduction in size and further increase in strength are also demanded for the parts, and much importance has been attached to the improvement in critical strength against pitting, which is one kind of rolling contact fatigue behaviors, (hereinafter, referred to as “pitting strength”) and wear resistance.
The steel material of low-alloyed steel for machine structural use that contains about 0.2 mass % of carbon and is used as a starting material for carburized parts and carbonitrided parts comes in a manganese based steel represented by SMn420, a manganese-chromium based steel represented by SMnC420, a chromium based steel represented by SCr420, and a chromium-molybdenum based steel represented by SCM420. The recent increase in prices of rare metal elements has been remarkable, and in particular, the price of molybdenum has increased sharply.
“Carbonitriding” includes “gas carbonitriding” in which ammonia gas is mixed in a carburizing atmosphere, whereby nitriding is performed simultaneously with carburizing, and it is said that nitrogen has an effect of increasing a resistance to softening during tempering. However, carbonitriding poses a problem of decreased hardening depth because nitrogen has a suppressive action on the diffusion of carbon, and in addition, nitriding treatment is performed at a lower temperature than carbonizing treatment. Further, because nitrogen is an austenite stabilizing element, and also lowers the Ms temperature in the same way as carbon, carbonitriding has a tendency to form substantial amount of retained austenite, which makes it difficult to obtain hard martensite.
Accordingly, the techniques for solving the problems with carbonitriding have been disclosed as “Gear having excellent tooth contact strength and manufacture thereof”, “High strength gear”, “Method for heat-treating carbo-nitriding treated member excellent in pitting resistance”, and “High hardness parts” in Patent Documents 1 to 4, respectively.